Sibling aggression in Eurasian lynx (Lynx lynx) occurs in half of the litters with 2-3 kittens (SOKOLOV et al.1994). cubs of both sexes can be aggressors and they attack cubs of the same or another sex. This sibling aggression coincides with the period when kittens start to take solid food at the age of 1.5 months. The relations of 15 cubs (7 males: 8 females) from 6 litters were studied in 2002-2003 in captivity at the biological station “Tchernogolovka”, near Moscow. Continuous data recording was used during visual observations. These observations were conducted for cubs between 1 and 6 months of age. Every week al kittens were weighted and blood sampling (1 ml) was conducted once per two weeks. Levels of testosterone, androstenedione and cortisol were measured by EIA in lynx cub plasma. Cubs 2-3 months of age had a clear peak of playful contact with the sibs. At this age it was the main type of social interactions with family members. Spontaneous fights of cubs were observed in 4 litters out of five with 2-3 kittens. The frequency of aggressive interactions was extremely low in these litters. However the intensity of aggression was quite high. Testosterone and cortisol didn’t show any clear pattern during the first six months of life. The level of androstenedione was at a maximum in cubs 42 days of age. In some kittens the androstenedione level clearly correlated with the dates of fights. Fight winners held the dominant position in the litter hierarchy which provided priority of access to available food resources. Even in captivity were food provided ab libitum the dominant cubs attained a higher body mass then than their sibs. One month after the period of sibling conflict the dominant cub had a more than 20% higher body mass than to the submissive cub. These differences may be more extreme when food supplies are low. Relations between cubs change significantly as a result of fights. Dominant sibs started to initiate more playful interactions than before.